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I was asked on a science test:'True or False: Molecules are moving all the time. I wrote false, because I thought molecules stopped moving at absolute zero. I was marked incorrect, and my teacher showed me that the book said molecules keep moving. Am I right? Do molecules stop moving at absolute zero?
Answer 1:

The quick answer to your question is no, molecules do not stop moving at absolute zero. They move much less than at higher temperatures, but they still have small vibrations at absolute zero. Sometimes classical definitions of absolute zero say that it is the temperature where all molecular motion ceases, but that definition is not technically correct.

Because molecules are very small, their movement is governed by the laws of quantum mechanics. Sometimes that means that they behave in ways that seem to be peculiar and unpredictable, because we are used to observing classical (large scale) physics in our daily lives.

When molecules move, they move in very specific ways. One type of molecular movement is vibration. The vibrations of the atoms and bonds are restricted because of the way quantum mechanics relates to their symmetry. Different types of vibrations have different energy levels. At higher temperatures, different modes of vibration (more vibrations) are available to the molecule. As molecules are cooled, they are able to vibrate in fewer and fewer ways, so movement slows approaching absolute zero. However, quantum mechanics has a strange feature called zero point energy. That means that even at its lowest energy state, a molecule does not have zero energy. Molecules, even at absolute zero, always have energy. The vibrational zero- point energy of the molecule still corresponds to movement, so even though the molecule is at absolute zero, it is still moving.



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